Despite concerns over a possible Axis invasion, orders for the general mobilisation of the Royal Yugoslav Army were not issued by the government until 3 April 1941, not to provoke Germany any further and precipitate war.

The Kingdom of Serbs, Croats and Slovenes was created with the merger of Serbia, Montenegro and the South Slav-inhabited areas of Austria-Hungary on 1 December 1918, in the immediate aftermath of World War I.

[3] Consequently, at the time World War II broke out in September 1939, the VKJ had several serious weaknesses, which included reliance on draught animals for transport, and the large size of its formations.

[7][8] The weaknesses of the VKJ in strategy, structure, equipment, mobility and supply were exacerbated by serious ethnic disunity within Yugoslavia, resulting from two decades of Serb hegemony and the attendant lack of political legitimacy achieved by the central government.

Fifth column activity was also a serious concern, not only from the Croatian fascist Ustaše and the ethnic German minorities but also potentially from the pro-Bulgarian Macedonians and the Albanian population of Kosovo.

Scholars have proposed several theories for the Royal Yugoslav Army's sudden collapse, including poor training and equipment, generals eager to secure a quick cessation of hostilities, and a sizeable Croatian nationalist fifth column.

Shortly after this, further attacks were made along the Drava between Ždala and Gotalovo in the area of the 27th Infantry Division Savska (27th ID) with the intention of securing crossings over the river, but they were unsuccessful.

These local attacks were sufficient to inflame dissent within the largely Croat 4th Army, who refused to resist Germans they considered their liberators from Serbian oppression during the interwar period.

[43] The following day, in the areas of the 42nd ID and Detachment Ormozki on the left flank of the 4th Army, the Germans cleared the territory north of the Drava, and border guard units were withdrawn south of the river.

[45] In the early evening of 7 April, German units in regimental strength began to cross the Drava near Barcs and established a second bridgehead in the sector of the 40th Infantry Division Slavonska (40th ID).

On that same day, Maček, who had returned to Zagreb after briefly joining Simović's post-coup d'état government, agreed to send an emissary to the 108th Infantry Regiment urging them to obey their officers, but they did not respond to his appeal.

[50] Following up the withdrawal of the divisional cavalry squadron, the Germans seized Suho Polje, west of Virovitica, cutting the main road to Slatina,[50] and the rebel Croat troops at Bjelovar made contact with them.

[51] At 11:15, Nedeljković arrived at divisional headquarters and shortly afterwards ordered the division to launch a counter-attack on the German bridgehead at Barcs at dawn the following day.

[56] On the first day of the invasion, LI Infantry Corps captured the Mura bridges at Mureck and Radkersburg (opposite Gornja Radgona) undamaged.

After having been grounded for most of the day by poor weather, in the afternoon the Yugoslav bombers flew missions against airfields and railway stations across the German frontier.

German and Italian air attacks interfered with the deployment of troops and command was hampered by reliance on civilian telegraph and telephone services.

[31] Despite this, German forces along the 7th Army front continued to push towards Ptuj, Maribor and Dravograd on 7 April, against significant resistance from border troops.

The German thrusts towards Ptuj and Maribor broke through the Yugoslav defensive line, but those advancing towards Dravograd were held up by border troops and a battalion of the 38th ID.

[63] On 8 April, disregarding orders from above, Palten led his kampfgruppe south towards Maribor, and crossed the Pesnica River in inflatable boats, leaving his unit vehicles behind.

[65] In view of German success, the Italian 2nd Army in northeastern Italy accelerated its preparations and issued orders for its V and XI Corps to conduct preliminary operations aimed at improving their starting positions for the planned attack on Yugoslavia.

However, Rupnik and the head of the operations staff of the headquarters of the 1st Army Group, Pukovnik Franjo Nikolić, hid the orders from Petrović and did not carry them out.

In the afternoon, the remaining elements of the 42nd ID also began to withdraw though Varaždinske Toplice to Novi Marof, leaving the Ustaše to take control of Varaždin.

[71] On the same day, Generalmajor Friedrich Kühn's 14th Panzer Division of XXXXVI Motorised Corps, supported by dive bombers, crossed the Drava at Zákány and drove southwest towards Zagreb on snow-covered roads in extremely cold conditions.

Nedeljković replied that he no longer had an army, and suggested that all Serb officers and men be ordered back to form a defensive line along the Sava and Una rivers.

On the same day, the German-installed interim Croatian government called on all Croats to stop fighting, and in the evening, LI Infantry Corps entered Zagreb and relieved the 14th Panzer Division.

[73] The 8th Panzer Division continued southeast between the Drava and Sava rivers, and meeting almost no further resistance, had reached the right flank of the 4th Army sector at Slatina by evening.

[72] Luftwaffe reconnaissance sorties revealed that the main body of the 7th Army was withdrawing towards Zagreb, leaving behind light forces to maintain contact with the German bridgeheads.

[80] To assist the Italian advance, the Luftwaffe attacked Yugoslav troops in the Ljubljana region, and the 14th Panzer Division, which had captured Zagreb on 10 April, drove west to encircle the withdrawing 7th Army.

[77] On 10 April, as the situation had become increasingly desperate throughout the country, Simović, who was both the Prime Minister and Yugoslav Chief of the General Staff, broadcast the following message:[15] All troops must engage the enemy wherever encountered and with every means at their disposal.

Don't wait for direct orders from above, but act on your own and be guided by your judgement, initiative, and conscience.On 12 April, the 14th Panzer Division linked up with the Italians at Vrbovsko, closing the ring around the remnants of the 7th Army, before thrusting southeast towards Sarajevo.